temperatures in the mountains

Where exactly do you mean?
It's the best time to visit the places in high altitudes since it's the hottest time of the year.

The plateau between Ardabil and Masuleh, the range between Nordooz and Ardabil including Mt Sabalan, as well as the Alborz Mountains, or rather the valley south of it, which seems to be at something like 1500 m. I would like to cross a mountain pass once in a while but mainly stay in the valleys right next to the mountains, which always seem to be at least 1000 m.
If I would sleep outdoors how cold would it be?

ُLate July and August will be the best time to roam around these places. You'll be fine. In some places it will be hot, In sabalan and Alborz (for example Firouzkooh) it might get cold, yesterday it was raining quite hard in Firouzkooh. If you are an experienced hiker you wont have a hard time. A warm cloth and a good sleeping bag will do in the outdoors if it gets cold. I don't think you will experience temperature below 12-15C (except in mountain tops like Sabalan)

Nopes - at most a bit chilly at night. During the day it can be a stove
which makes a cooler night a downright gift.

Good luck!